[Qgis-developer] Expression based labeling

[Sandro Santilli]

On Fri, Oct 28, 2011 at 01:53:53PM -0300, Martin Dobias wrote:
> On Fri, Oct 28, 2011 at 1:33 PM, Sandro Santilli <strk at keybit.net> wrote:
> > On Fri, Oct 28, 2011 at 12:23:24PM -0300, Martin Dobias wrote:
> >> On Fri, Oct 28, 2011 at 4:15 AM, Sandro Santilli <strk at keybit.net> wrote:
> >> > Would it make sense to abstract provider-specific IDs so that the
> >> > [0, N-1] range holds for everyone ?
> >>
> >> This is actually done within attribute table - we store feature IDs
> >> for the range [0,N-1] so that we can later access the feature data by
> >> row number. But that comes at an additional price: the layer has to be
> >> scanned first in order to obtain the mapping and it has to be updated
> >> whenever some changes to the layer are done.
> >>
> >> What would be the requirements of such provider-specific IDs? What to
> >> do when a feature is deleted? If we recreate the mapping, some IDs
> >> stored by users would change, leading to problems if the user stores
> >> them as references to features. If we keep the original mapping, then
> >> it may contain holes and therefore the convenient [0, N-1] range would
> >> not be valid.
> >
> > What happens if you delete a feature from a shapefile ?
> 
> Apparently OGR only marks the feature as deleted in DBF, other IDs are
> not changed.

Note that one advantage of delegating to providers setting up a mapping
is we could finally drop the requirement of an integer identifier in
postgresql. There are real-world cases in which primary key is a text
identifier.

--strk;

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